Purification of gelatin



Patented July 8, 1947 PURIFICATION OF GELATIN John Alfred Henry Hart andEdward William Lee, Ilford, England, assignors to Iford Limited, Ilford,England, a British company No Drawing. Application May 19, 1943, SerialNo. 487,664. In Great Britain June 24, 1942.

Claims.

This invention relates to the purification of gelatin.

It is well known that normal gelatin as received from the manufacturersmay contain impurities which on burning the gelatin away leave an ashwhich may be as much as 2% or even more of the weight of the gelatin.These impurities, with others which may be present initially but do notappear in the ash content, may have undesirable effects on the behaviourof the gelatin during subsequent use.

Th present invention is concerned with a method of reducing the quantityof such impurities in gelatin. It is found that the purified gelatinobtained is of especial value in the production of clear photographicgelatin layers such as substrata, supercoat layers, filter layers andantihalation layers, in the production of gelatinosilver halide emulsionlayers, and in general in all cases where photographic layers having abasis of gelatin are required. Moreover, the purified gelatin is ofespecial value in the production of light filters which consist of dyedgelatin sheets.

According to the present invention the quantity of undesirableimpurities present in the gelatin is reduced by treating the gelatinwith a solution, in a mixture of water and a watermiscible organicnon-solvent for the gelatin, of an acid which forms, with the cations ofimpurities present in the gelatin, salts which are soluble to asubstantial extent in the said solution. The concentration of acidemployed is preferably such that during the treatment the gelatin isbrought to its isoelectric condition.

A mixture of water and a water-miscible organic non-solvent for thegelatin serves to permit some swelling of th gelatin, which facilitatesthe purification, whilst preventing undue swelling which might make thepurified product difficult to handle. A very convenient mixture toemploy is a mixture of water and ethyl alcohol. In such a mixture thealcohol should preferably not be less than about 30% or undue swellingwill occur and not more than about 70% or insuflicient swelling willtake place. A mixture of 50 parts water and 50 parts of 95% alcohol byvolume is quit satisfactory. Other water miscible lower alcohols, e. g.methyl alcohol, n-propyl alcohol and isopropyl alcohol and other watermiscible organic non-solvents for the gelatin e. g. acetone, dioxane andethylene glycol monoethyl and monomethyl ethers, may also be employed.The term lower alcohol used herein is to be understood as including onlyalcohols containing 1 to 3 carbon atoms,

As indicated above the quantity of acid employed is preferably such thatthe gelatin is brought to its isoelectric condition. In general therequired quantity of acid will vary from one gelatin to another as aresult of differences in the conditions of manufacture of the gelatin.In general, the concentration of acid employed may be such that thegelatin is brought to a, pH below that of its isoelectric condition, ifthe acid chosen is such that a small residue of it remaining in thegelatin will have no harmful effect on the desired behaviour of thegelatin. If the gelatin is required to be free from residual acid of anykind, the treatment with acid solution containing more acid than isnecessary to bring it to its isoelectric condition may be followedimmediately by treatment with a mixture of water and organic nonsolventcontaining no acid.

A large number of acids is available any of which may be employed eithersingly or as mixtures to satisfy the conditions above set forth.Examples ar hydrochloric, nitric acetic, lactic, maleic, benzoic,phthalic, benzene-sulphonic, phenol-sulphonic, salicyl-sulphonic andsallcyclic acids. Sulphuric and phosphoric acids are somewhat lesseffective when the gelatin contains cations of the alkaline earth group,as the sulphates and phosphates of such metals are relatively insoluble.

The purification can most; readily be effected by stirring the gelatin,preferably in powder or flake form, in the solution defined above, atnormal room temperature. However, the treatment may be effected athigher or lower temperatures. The treatment may be effected for periodsvarying from a few minutes up to an hour or more as desired.

EXAMPIE 1 First treatment secend'treament 'Ih'e sieve and its contentswere then removed, allowed to drain, and then stirred with two separatequantities of 1 litre of -99% ethyl alcohol for 30 minutes each. Thegelatin was then allowed to drain thoroughly, spread out on muslin andallowed to dry overnight.

4 By keeping the swelling of the gelatin low the purified gelatin can bedrained rapidly and dried without elaborate plant. Moreover thetreatment leaves the gelatin substantially unchanged in yielded apurified gelatin of ash content about 5 form and in a condition in whichit is readily 0.01% when treated by this process. soluble in the usualgelatin solvents. Little attention is required to operate the process,it may EXAMPLE 2 i be operated at normal room temperatures, and Thisexample illustrates variation in the nature no accurate temperaturecontrol is necessary of the acid emp oyed. The proce r lndlcated Thealcohol or other organic non-solvent for the in Example 1 Was followedus t e I W gelatin which is used in the process may readily acids in thequantities stated. be recovered.

As already indicated, gelatin prepared by the A 7 Q i y f cid d issg lvedinw litres process of this invention is of especial value in ggfifiiggf g f the production of clear photographic gelatin layers. Inparticular, the purified gelatin is of esg cw soiufim pecial valule as abasis for the substraitla wlbilch SO are norma ly applied tophotographic lm ase 1 s 6 before the photographic emulsion or othergelatin g gg i; 51 20 layer is coated in order to secure satisfactoryad- Phthalic acid: .III 9 gins. hesion between the film base and theemulsion or Benzene sulphonic acid. 30 cos. of 307 aqueous solution.Phenol sulphonic acid..." 30 cos. of 30%; aqueous solution. Othel:gelatm layer Salicylsulphonic acid e gms. It is found that gelatinpurified by the process of this invention may be dissolved to form solu-In all cases a very effective purification of the b10115 Whwh, o coatmgon film base made from gelatin was obtained organ c colloids such ascellulose esters, and dry- E 3 ing, yield substrata capable of givingconsiderably XAMPLE improved adhesion of gelatin layers to the film Thisexample illustrates variations in the nabase. The purified gelatin isespecially valuable ture and proportions of the organic non-solvent inthis connection in forming substrata on celemployed and in thetemperature and time of lulose acetate film base. Solutions used forcoattreatment. The procedure followed was as in ing such substrata arereferred to hereinafter Example 1, and the variations from that exampleas subbing solutions. are set out in the following table: In theproduction of a purified gelatin intended Example No 3a 3b 3c 3d 32 3f30 3h 31' 1st Treatment Quantity of gel., 100 100 100 100 150 15o 100100 100.

I115. N n-Solvent Acetone Acetone..- Methyl Methyl Methyl Ethyl alco-Ethylene gly- Dioxane. Ethyl alcocohol. alcohol alcohol hol 99%. colmono hol95%.

methyl ether. Volume of non- 5 6 5 6 9 9 3 2% 6.

solvent, litres. Volgrme of water, 5 4 5 l. 4 l 1 2 2% 4.

1 I05. Acid employed... Salicylic Salicylic. Sa1icylic Salicylic".Acetic..." Salicylic..." Phthalic H ydr o l-methyl chloric tridecylSulphuric. Quantityofacid 5gms.. 5gnis 5gms 5gms. l0ccs 7.5gms lOgrns 30cos. N/l 20 25% S0 I1. Time of treat- 4 4 4 4 6% 1 2 ment, hrs. Temp. oftreat- 65 e5 65 75 115 o5 70.

ment, F.

2d Treatment Non-Solvent Ethylalc Ethylalc. Ethylalc. Ethylalc None NoneNone Dionne..." Ethyl ale.

95%. 95%. 95%. 95%. 95%. Volume of non- 1 1.4.

solvent, litres. Volume of water, 0 0 0 0 0.2 0.6.

litres. Time of treat- $6 M l.

nient, hrs. Temp. of treat- 65 65 65 65 65 70.

ment, F

In all cases the gelatin obtained had a much reduced ash content.

The purification procedure indicated above and illustrated in theforegoing examples presents 65 many advantages. Thus the gelatin doesnot require any special preliminary treatment such as dissolving andshredding, but may be treated in the form of thin leaf, flake or powder,in which forms it is usually supplied by the manufacturers. 70

for use as the basis of photographic substrata it is preferred to employan organic acid, and particularly salicylic acid, for the purificationsince organic acids are preferred as stabilisers for the subbingsolution. In such a case the gelatin may be brought during the treatmentto a pH lower than that corresponding to its isoelectric condition, as asmall residue of the organic acid has no harmful effect on the behaviourof the gelatin in a subbing solution.

The dispersions in organic liquids of the purified gelatin produced bythis invention, such as subbing solutions, are of reduced opalescenceand possess greater stability compared with similar solutions obtainedfrom untreated gelatin. It is possible to obtain clear, i. e.non-opalescent, subbing solutions which contain a substantially higherproportion of gelatin, e. g. 3% or 4%, than can usually be obtainedusing untreated gelatin. Moreover, the purified gelatins of the presentinvention make possible the production of subbing solution which containa smaller proportion of stabilising acid than is usual in suchsolutions. Furthermore the purified gelatin can be used to formulatestable subbing solutions containing higher proportions Of non-hydroxylicsolvents, e. g. acetone, compared with the proportion possible whenusing unpurified gelatin.

The following are examples of solutions suitable for forming a subcoaton photographic film base.

A. For various cellulose organic ester film bases:

Gelatin gms. 10 gms. Nitrocellulose 2 gins. Salicylic acid l 2 gins. or2.5 gms Distilled water 30 ccs. 3O ccs Methyl alcohol 370 ccs. 60 cos.Acetone -l 600 ccs. 900 ccs. B. For cellulose nitrate film base:

Gelatin grams 5 Salicylic acid d0 1 Distilled Water cubic centimeters-2O Ethyl alcohol do. 780 Acetone do- 200 In addition to its use insubstrata, the purified gelatin of this invention may, as indicatedabove, be employed in the production of other photographic layers havinga basis of gelatin, e. g. gelatino-silver halide emulsion layers, filterlayers, supercoat layers and the like. It presents a particularadvantage in the manufacture of dyed filter layer since by its use it ispossible to obtain such layers almost entirely free from haze, even whenusin dyes which are particularly prone to form precipitates (resultingin haze) with most untreated gelatins.

What we claim is:

1. A process for purifying gelatin which comprises treating the gelatinin finely divided form with a solution of an acid in a mixture of waterand a water-miscible organic non-solvent for the gelatin, said acidbeing one which forms, with the cations of the impurities in thegelatin, salts which are soluble to a substantial extent in the saidsolution, at a pH not above the isoelectric point of the gelatin, saidmixture containing a substantial amount of non-solvent and insufficientwater to produce substantial swelling of the gelatin, and separating thesolution from the gelatin.

2. A process for purifying gelatin which comprises treating the gelatinin finely divided form with a solution of an acid in a, mixture of waterand a water-miscible organic non-solvent for the gelatin, said acidbeing one which forms, with the cations of the impurities in thegelatin, salts which are soluble to a substantial extent in the saidsolution, said acid being present in such concentration that during thetreatment the gelatin is brought to the isoelectric condition, aidmixture containing a substantial amount of nonsolvent and insufilcientwater to produce substantial swelling of the gelatin, and separating thesolution from the gelatin.

3. A process for purifying gelatin which comprises treating the gelatinin finely divided form with a solution of an acid in a mixture of waterand a water-miscible alcohol, said acid being one which forms, with thecations of the impurities in the gelatin, salts which are soluble to asubstantial extent in the said solution, at a pH not above theisoelectric point of the gelatin, said mixture containing a substantialamount of water-miscible alcohol and insufiicient water to producesubstantial swelling of the gelatin, and separating the solution fromthe gelatin.

4. A process for purifying gelatin which comprises treating the gelatinin finely divided form with a solution of an acid in a mixture of waterand a water-miscible alcohol, said acid being one which forms, with thecations of the impurities in the gelatin, salts which are soluble to asubstantial extent in the said solution, said acid being present in suchconcentration that during the treatment the gelatin is brought to theisoelectric condition, said mixture containing a substantial amount ofwater-miscible alcohol and insufiicient water to produce substantialswelling of the gelatin, and separating the solution from the gelatin.

5. A process for purifying gelatin which comprises treating the gelatinin finely divided form with a solution of an acid in a mixture of waterand a water-miscible organic non-solvent for the gelatin, said acidbeing one which forms, with the cations of the impurities in thegelatin, salts which are soluble to a substantial extent in the saidsolution, at a pH not above the isoelectric point of the gelatin, saidmixture containing a substantial amount of non-solvent and insufilcientwater to produce substantial swelling, and thereafter washing thegelatin with a mixture of water and a water-miscible organic non-solventfor the gelatin which does not contain any such acid.

6. A process for purifying gelatin which comprises treating the gelatinin finely divided form with a solution of salicylic acid in a mixture ofwater and a water-miscible organic non-solvent for the gelatin, at a pHnot above the isoelectric point of the gelatin, said mixture containinga substantial amount of non-solvent and insufiicient water to producesubstantial swelling of the gelatin, and separating the solution fromthe gelatin.

7. A process for purifying gelatin which comprises treating the gelatinin finely divided form with a solution of salicylic acid in a mixture ofwater and a water-miscible organic non-solvent for the gelatin, saidmixture containing a substantial amount of non-solvent and insufiicientWater, the salicylic acid being present in such concentration thatduring the treatment the gelatin is brought to that isoelectriccondition.

8. A process for purifying gelatin which comprises stirring gelatin inthe form of powder in a solution of salicylic acid in a mixture of waterand a saturated aliphatic alcohol of l to 3 carbon atoms at a pH notabove the isoelectric point of the gelatin, said mixture containing asubstantial amount of a non solvent and insufiiicent water to producesubstantial swelling of the gelatin, and separating the solution fromthe gelatin.

9. A process for purif ing gelatin which comprises stirring gelatin inthe form of powder in a solution of salicylic acid in a mixture of waterand a saturated aliphatic alcohol or 1 to 3 carbon atoms, said mixturecontaining a. substantial amount of alcohol and insufiicient water toproduce substantial swelling of the gelatin, and the salicylic acidbeing present in such concentration that during the treatment thegelatin is brought to the isoelectric condition.

10. A process for purifying gelatin which com- REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PA'I'ENTS Number Name Date 1,457,466 Smith June 5, 19232,191,206 Schwartz Feb. 20, 1940 FOREIGN PATENTS Number Country Date153,526 Great Britain NOV. 11, 1920 OTHER REFERENCES Proteins and TheirTheory of Colloidal Behavior, Loeb. McGraw-Hill Book Company, 1stedition, 1922 (pp. 253-255).

